The present invention is related to hearing systems and, more particularly, to implantable microphone devices that may be utilized in hearing systems.
Conventional hearing aids are placed in the ear canal. However, these external devices have many inherent problems including the blockage of the normal avenue for hearing, discomfort because of the tight seal required to reduce the squeal from acoustic feedback and the all-too-common reluctance for hearing-impaired persons to wear a device that is visible.
Recent advances in miniaturization have resulted in the development of hearing aids that can be placed deeper in the ear canal such that they are almost unnoticeable. However, smaller hearing aids inherently have problems, which include troublesome handling and more difficult care.
Implantable hearing devices offer the hope of eliminating problems associated with conventional hearing aids. One requirement for a fully implantable hearing device or system is an implantable microphone.
All microphones necessarily contain an interface between the internal components and the environment in which it will be situated. For non-piezoelectric designs, air-conduction microphones utilize a membrane, which can be made of various materials, stretched or formed to varying tensions. The tension in the membrane has a first order effect on the response of the microphone. A highly stretched membrane will tend to resonate at a high frequency, with a flat response at frequencies below the resonance. However, a higher tension in the membrane will also tend to lower the sensitivity of the microphone.
Prior art implantable microphones for use with hearing systems have comprised an electret microphone disposed within an air cavity, enclosed by a stretched stainless steel membrane. The air cavity is hermetically sealed, necessitated by implantation in the body. The membrane is stretched tight and laser welded; the resulting system frequency response therefore has a low sensitivity and a sharp high frequency resonance peak. An improved device response would have high sensitivity, comparable to an electret microphone alone in air, and would be generally flat across the audio frequency, especially in the range of speech (500-4,000 Hz). Additional requirements for an improved implanted microphone include low distortion and low noise characteristics.
Traditional, non-implantable type microphones have an air cavity behind the membrane that is not sealed, with reference to the nearest surface behind the membrane. Traditional microphones are concerned with optimal membrane displacement, and typically have several air cavities which are used to influence the shape of the microphone response. An implantable microphone design that incorporates a membrane, enclosing a sealed chamber containing an electret microphone, is necessarily concerned with an optimal pressure build-up in the sealed cavity. This pressure build-up in turn displaces the membrane of the electret microphone. However, a sealed air cavity presents new challenges to the design and optimization of implantable microphones.
With the advent of fully implantable devices for stimulating hearing, there is a great need for implantable microphones that provide excellent audio performance. The present invention provides improved audio performance through improvement of microphone design.